1. Field of the Invention
The present invention relates to a constant voltage circuit having a resistor circuit in a closed loop formed by connecting the output terminal and a load. The invention also relates to a constant voltage/constant current changeover circuit having a common output terminal for outputting a constant voltage and a constant current, wherein a constant voltage output section is connected through a resistor to an output terminal.
2. Description of the Prior Art
In electrophotographic image forming devices, such as a laser beam printer, copying machine and facsimile device, a photosensitive drum and a transfer roller are disposed in confrontation with each other with a sheet passage therebetween. In forming an image on a recording sheet, a toner image formed on the surface of the photosensitive drum is transferred onto the recording sheet. This is done by applying forward bias voltage (transferring bias voltage) between the photosensitive drum and the transfer roller. After the recording sheet has passed through a gap between the photosensitive drum and the transfer roller, reverse bias voltage (cleaning bias voltage) is applied between the photosensitive drum and the transfer roller to return the residual toner on the transfer roller back to the photosensitive drum.
More specifically, when transferring the toner image onto the recording paper, transferring bias voltage is applied between the photosensitive drum and the transferring roller so that the former is at a ground potential and the latter is at a negative potential. When returning the residual toner back to the photosensitive drum, cleaning bias voltage is applied so that the transferring roller is at positive potential. The photosensitive drum is maintained at the ground potential. A constant current control is executed to apply the transferring bias voltage to uniformly transfer the electrically charged toner particles. On the other hand, a constant voltage control is executed to apply the cleaning bias voltage.
FIG. 1 shows a conventional constant voltage/constant current changeover circuit 30 which can selectively output the transferring bias voltage and cleaning bias voltage to be applied between the photosensitive drum and the transferring roller through the common output terminals. The circuit 30 includes a first circuit 11, a second circuit 12, a resistor R1 connected between the outputs of the first circuit 11, a pair of voltage division resistors R2 and R3 connected in series between the outputs of the second circuit 12, a comparison operation circuit 14, a first oscillation control circuit 16 and a second oscillation control circuit 18. The first circuit 11 is capable of controlling an output voltage. The second circuit 12 is capable of controlling an output current. A voltage VT developed across the resistor R3 is applied to the comparison operation circuit 14 which compares the voltage VT with a reference value. An output from the comparison operation circuit 14 is applied to both the first and second oscillation control circuits 16 and 18. The first oscillation control circuit 16 controls the first circuit 11 in accordance with the output from the comparison operation circuit 14. The second oscillation control circuit 18 controls the second circuit 12 in accordance with the output from the comparison operation circuit 14.
The high voltage output terminals of the first and second circuits 11 and 12 are connected, and the low voltage output terminal of the first circuit 11 is connected through an output terminal O to the transfer roller 5. The low voltage output terminal of the second circuit 12 is connected to ground as is the case in the photosensitive drum 1. Hereinafter, the photosensitive drum 1 and the transfer roller 5 will be referred to as "a load L".
The first circuit 11 includes a transformer T1, a transistor Q1, and a voltage doubling/rectifying circuit configured by capacitors C11 and C12, diodes D11 and D12, and a resistor R11. The transformer T1 consists of a primary winding L11, a secondary winding L12, and an auxiliary winding L13. The transistor Q1 has a bass connected to one terminal of the auxiliary winding L13, an emitter connected to ground, and a collector connected through the primary winding L11 to a voltage source VDD which supplies 24 volts in this instance. The transistor Q1, the auxiliary winding L13 and the first oscillation control circuit 16 form a blocking oscillation circuit which is well known in the art. Depending on the operation condition of the transistor Q1, the current does not flow or flows in the primary winding L11. The voltage doubling/rectifying circuit is also well known in the art, which rectifies the current flowing in the secondary winding L12 and also doubles the voltage developed across the secondary winding L12.
The second circuit 12 includes a transformer T2, a transistor Q2, and a half-wave rectifying circuit configured by a diode D21 and a capacitor C21. The transformer T2 consists of a primary winding L21, a secondary winding L22, and an auxiliary winding L23. The transistor Q2 has a base connected to one terminal of the auxiliary winding L23, an emitter connected to ground, and a collector connected through the primary winding L21 to the voltage source VDD. The transistor Q2, the auxiliary winding L23 and the second oscillation control circuit 18 form a blocking oscillation circuit similar to that provided in association with the first circuit 11. Depending on the operation condition of the transistor Q2, the current does not flow or flows in the primary winding L21. The half-wave rectifying circuit rectifies and smoothens the current flowing in the secondary winding L22.
The first oscillation control circuit 16 operates only when a selection signal S1 is applied thereto. Similarly, the second oscillation control circuit 18 operates only when a selection signal S2 is applied thereto. When the voltage developed across the resistor R3 is greater than the reference value set in the comparison operation circuit 14, the first and second oscillation control circuits 16 and 18 prolong the oscillation periods in the respective blocking oscillation circuits. Conversely, when the voltage developed across the resistor R3 is smaller than the reference value set in the comparison operation circuit 14, the first and second oscillation control circuits 16 and 18 shorten the oscillation periods in the respective blocking oscillation circuits. Electric power supplied to the secondary windings L12 and L22 is therefore changed so that the detection voltage VT is brought into coincidence with the reference value.
The comparison operation circuit 14 can change the reference value to be compared with the detection voltage VT depending on the selection signals S1 and S2.
The constant voltage/constant current changeover circuit 30 forms a constant current circuit as shown in FIG. 2(a) when the selection signal S1 is applied to the first oscillation control circuit 16, causing the first circuit 11 to operate and the second circuit 12 not to operate. In this case, no current flows in the second circuit 12 due to rectification of the diode D21. The constant current circuit shown in FIG. 2(a) includes the first circuit 11, comparison operation circuit 14, oscillation control circuit 16 and resistors R1 through R3. A current equal to a load current IL flows in the resistor R3. Hence, the detection voltage VT that is proportional to the load current IL in developed in the resistor R3. Accordingly, the load current IL is maintained constant due to the control of the first oscillation control circuit 16.
On the other hand, when the selection signal S2 is applied to the second oscillation control circuit 18, the first circuit 11 does not operate but the second circuit 12 operates. In this case, due to the rectification of the diodes D11 and D12, no current flows in the first circuit 11. Accordingly, a constant voltage circuit as shown in FIG. 2(b) is provided. The constant voltage circuit shown therein includes the second circuit 12, comparison operation circuit 14, second oscillation control circuit 18 voltage division resistors R2 and R3. That is, a detection voltage VT proportional to the output voltage Vc output from the second circuit 12 is obtained, the output voltage Vc of the second circuit 12 becomes constant due to the control of the second oscillation control circuit 18.
However, the voltage Vc output from the second circuit 12 is applied to the load L through the resistor R1, causing a voltage drop (R1.times.IL) therein. This results in variation in the voltage applied to the load L depending on the load current IL. The voltage VO applied to the load L is given by the following equation. EQU VO=Vc-R1.times.IL (1)
The variation in the voltage VO can be suppressed during the constant voltage operation if the resistor R1 with a small resistance is used. However, the small resistance increases Ohmic dissipation during the constant current operation. Therefore, the resistance of resistor R1 should not be so small.
The above-noted problem exists not only in the constant voltage/constant current changeover circuit 30 of the type in which the constant voltage output and a constant current output are applied through the common terminal but also in a constant voltage circuit configured as shown in FIG. 2(b). Because if a resistive element is present on a line connecting the output of the constant voltage circuit to the load L, the voltage VO applied to the load L will vary depending on the load current IL.
FIG. 3 shows a conventional constant voltage circuit 40. The circuit shown therein differs from the circuit shown in FIG. 2(b) in that in lieu of inputting the output of the secondary winding L22 to the comparison operation circuit 14, a second auxiliary winding L24 is provided in the transformer T2. In addition, a half-wave rectifying circuit consisting of a diode D31 and a capacitor C31, and and voltage division resistors for voltage dividing the output of the half-wave rectifying circuit are connected to the second auxiliary winding L24. The output obtained from the resistors R31 and R32 is applied to a comparison operation circuit 14 as a detection voltage VT. This constant voltage circuit 40 thus configured can only set the voltage Vc developed between the two terminals of the secondary winding L22 to a constant value. However, if a resistor circuit Rx is interposed in the line for applying the output of the constant voltage circuit 40 to the load Lx, the voltage VO applied to the load Lx varies depending on the load current Ilx, so that the voltage VO applied to the load Lx cannot be made constant.